The invention relates to detecting the position of a conveyor belt, light beams being directed onto the region of an edge of the conveyor belt and a jump in intensity of the light beam as a consequence of the partial insertion of the conveyor belt into the beam path being detected and evaluated in order to determine and control the position of the conveyor belt.
Methods and devices of the said type are disclosed, for example, in WO 99/00709. In this document, a source for pulsed light or a scanned or pulsed laser beam is arranged on one side of a belt, and a photoreceiver is arranged on the opposite side, in order to detect the position of the belt. Furthermore, this document describes a control system which controls the belt on the basis of detecting its position. In the case of this technical solution, the absorption of the light by the belt is detected and evaluated to determine the position of the belt. However, this functions only when the absorption of the fight beams by the belt is great enough to produce a jump in intensity of the light beam by the insertion of the belt which is clearly delimited and therefore sufficient for detecting the position. This is not ensured in the case of many belts or other materials whose position is to be detected, since these are entirely or partially transparent. For example, belts which transport printing materials for electrophotographic imaging frequently consist of transparent material, since the latter has a multiplicity of properties desired for the said application. However, the position of transparent material also has to be detected and, if appropriate, corrected in other technical fields, for example in the manufacture or further processing of transparent plastic webs, plastic sheets or glasses. In many applications, the arrangement of nontransparent strips on the material is impossible or expensive and frequently unreliable, in particular stuck-on strips can come off.
The invention is therefore based on the object of the method and the device and control such that the position of transparent material can also be detected with high accuracy. This object is achieved by virtue of the fact that the light beams are polarized light and the beam path is directed against bounding surfaces of the transparent conveyor belt in such a way that at least a partial reflection occurs and the jump in intensity caused thereby is detected. The object is further achieved by virtue of the fact that the light source is a light source for polarized light, that the light source is directed onto bounding surfaces of the transparent conveyor belt in such a way that at least a partial reflection occurs, and that the receiver detects the jump in intensity caused by the reflection, and the evaluation device evaluates the jump in intensity in order to display the position of the conveyor belt. The object is yet further achieved by virtue of the fact that the light source is a light source for polarized light, that the light source is directed onto bounding surfaces of the transparent conveyor belt in such a way that at least a partial reflection occurs, and that the receiver detects the jump in intensity caused by the reflection, and whose position serves the evaluation and control device for the purpose of controlling the position of the conveyor belt.
The advantage of the invention is that a material, which can be sheets or belts, can have its position detected even when it does not have sufficient absorption of light. Since it is also possible to detect other nontransparent materials, the method and the device can be used in a substantially more versatile fashion because one of the specific properties of the conveyor belt material is that a material edge can be detected virtually independently. This results from the fact that use is made only of the reflection properties of the material for polarized light. However, these are a function only of the angle of irradiation and the refractive index of the conveyor belt material. The angle of irradiation required for a reflection can be calculated via the Fresnel formulae. It is essential in this case to select an angle of irradiation for which the fraction of the reflection is so high that a sufficiently high jump in intensity is achieved. This method functions even in the case of a degree of soiling of the conveyor belt material, and in some refinements the jump in intensity is even further increased by soiling. All types of light can be used, that is to say also infrared or ultraviolet.
The jump in intensity can be detected in various ways. The reflected light can be detected, or it is possible to detect the light not reflected by a bounding surface of the conveyor belt material. In both cases, it is the jump in intensity which was caused by the reflection that is detected. Of course, it is also possible to combine the two with one another. The angle of irradiation xcex1 of the light onto the bounding surface is expediently selected in such a way that a high degree of reflection is achieved. A possible working range is an angle of irradiation xcex1 of between 40 and 80xc2x0, the angle of irradiation xcex1, however, preferably being at least 60xc2x0. In order to achieve a jump in intensity which can be effectively detected, it is proposed to use linearly polarized light. The light is advantageously polarized perpendicular to the plane of incidence, since this is reflected more strongly than light polarized parallel to the plane of incidence.
If the beam is guided such that the jump in intensity is not detected until after a plurality of reflections at the bounding surfaces, a multiplication of the effect and a thus a substantial amplification of the jump in intensity are achieved. Many refinements are possible in order to achieve the above-named effects. The following embodiments serve the purpose of implementing these and further functional principles.
One embodiment provides that the light source and the receiver are arranged on one side of the conveyor belt material. In this case the arrangement of the receiver corresponds to the angle of reflection of the reflected light, in order to receive the latter. Since the beam is reflected both at the conveyor belt material surface, that is to say the interface between air and material, and at the lower a interface between material and air, it is expedient for the receiver to be arranged such that it receives the reflected light beams from both bounding surfaces of the conveyor belt material. In this way, more light is reflected and the jump in intensity is increased. This is independent of whether the receiver detects the reflected light or whether it is arranged such that it detects the light not reflected by bounding surfaces.
One advantageous development provides that a further receiver is arranged on the side of the conveyor belt material opposite the light source in such a way that it receives the light passing through the material. This embodiment has the advantage that it is possible to detect the reflection by one receiver and the transmission by another receiver. In this way, it is also possible to determine soiling of the material and/or the presence of scratches. The advantage of this embodiment is that the position of the edge can also be determined with the aid of the transmission receiver in conjunction with a greatly depressed reflectivity of the material. Furthermore, the detection both of the reflectivity and of the transmission is suitable for the purpose of outputting a corresponding signal in the case of excessively soiled material, in order to be able to eliminate such interference if appropriate.
A further refinement provides that the receiver is arranged on the opposite side of the conveyor belt material for receiving the non-reflected light. In this case, it can detect the jump in intensity by virtue of the fact that it detects both the light passing the edge of the material and the substantially weaker light produced by the reflection and, if appropriate, also an additional absorption. Of course, the jump in intensity can also be detected in the case of total reflection, indeed even more clearly then. This refinement is very insensitive to soiling of the material, since although said soiling possibly reduces the reflectivity of the material, it also simultaneously reduces the transmission, with the result that it is possible, as before, to detect ajump in intensity at the edge of the material. This receiver can also be combined with a receiver for the reflected light.
A further refinement provides that the light source is arranged on one side of the conveyor belt material and a reflector is inserted into the beam path on the other side, a receiver being arranged in such a way that it detects the beam path reflected by the reflector, but does not detect a beam path reflected by a bounding surface. In the case of such a refinement, a substantial increase in the jump in intensity is achieved by virtue of the fact that the receiver is arranged on the same side of the material as the light source and specifically in such a way that the jump in intensity is amplified by two-fold passage of the beam through the material in the region of the edge. This arrangement can be configured in such a way that the light source and the receiver form, with the reflector, a triangle which is produced by the angle of irradiation and angle of reflection of the light at the reflector, or it can be provided that the reflector is arranged in such a way that the beams reflected by it run parallel to the incident beams. In both exemplary embodiments, the beam must pass the bounding surfaces four times, and on each occasion there is a reflection which amplifies the jump in intensity.
A further advantage of this refinement is that the receiver receives both the fraction of the beams which passed the conveyor belt material edge, that is to say have not struck any bounding surface of the material, and the fraction which still passed through the material despite the four-fold reflection. Since both values can be detected, it is also possible to detect soiling of or scratches on the material, since these reduce the reflectivity, and the transmission fraction is thereby increased. Moreover, by using both measured values, it is possible to achieve a correction of the individual values, for example, by averaging and a higher accuracy can be achieved in this way. As a result, in the case of this refinement, it is possible to achieve the same advantages as in the case of the embodiment having two receivers, a reflection receiver and a transmission receiver. In the case of the refinement in which the reflector retroreflects the reflected beams parallel to the incident beams, it is possible that the light source and the receiver form a functional unit. It can then expediently be provided that both are located in one housing. The reflector can be designed in such a way that it rotates the direction of polarization of the light by 90xc2x0.
The light source can be, for example, a point light source whose emission characteristic is adapted to the conditions, if appropriate, by arranging suitable optics, for example, stops, upstream. The sensitivity of the measuring method with respect to a change in spacing of a transparent conveyor belt relative to the transmitter and receiver is a function of the type of illumination. However, it is expediently provided that the light source emits parallel light beams, the light beams extending in a region transverse to the conveyor belt edge. Such a parallel beam path has the advantage that fluctuations in the spacing of the material surface from the transmitter and receiver do not feature as errors in the measurement. Of course, it must be ensured that the light images the region of the edge of the material on the receiver. For this purpose, expected fluctuations in the position of the edge are to be included when calculating the dimensions of the region. Another possibility is that the light source emits a scanned beam which sweeps over a region which extends transverse to the edge in such a way that the edge is detected in the case of expected fluctuations in its position. It is also possible in the case of the scanned beam for the latter to be moved in parallel.
The receiver is expediently such that it extends over a region which corresponds to the emitted light beams. A photoreceiver, for example a diode linear array, can be provided as a receiver.
In order to take account of variations, for example soiling of the conveyor belt, a controller can be provided for controlling the intensity of the light beams emitted by the light source, the input variables being the intensities of the received light beams before and after the jump in intensity, and the control aim being the improvement of the jump in intensity. It is possible, furthermore, that the controller activates a display, which produces a signal, when the reflection of the light beams by the material no longer suffices for the exact determination of the position of the edge.
An additional function can be integrated into the device by a planar receiver, as a result of which it is possible to determine an oblique position of the conveyor belt material edge or by detecting the position of the edge at two spaced apart regions and determining an oblique position of the edge therefrom.
An additional function is also possible for the control device by virtue of the fact that at least one receiver determines the data on the oblique position of the edge of the conveyor belt material and feeds them to the control device, the latter being such that it undertakes a correction of the oblique position.
Of course, all functional refinements can be implemented both by the embodiments proposed and by further ones, both in the case of the device and in the case of the control device.